EDITED 1/8/16: A contour interval is a the elevation difference between two adjacent contour lines.
Previous person put some ridiculous answer. This one is the correct answer on APEX and does not need improvement.
convection
When we divide the mass of a substance by its volume we get its density.
remote sensing
North -Apex :)
less-dense air has a higher temperature
You are unlikely to find a horizon on a map. The horizon (or skyline) is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not.
full moon
distortion